Current transformer having an accuracy unimpaired by stray flux from adjacent conductors



June 10, 1969 F. STEEN 3,449,703

CURRENT TRANSFORMER HAVING AN ACCURACY UNIMPAIRED BY STRAY FLUX FROM ADJACENT CONDUCTORS Filed March 20, 1968 //v va/vro/v: FLOYD L. \STL'EN,

By 7km A TTORNE) CURRENT TRANSFORMER HAVING AN AC- CURACY UNIMPAIRED BY STRAY FLUX FROM ADJACENT CONDUCTORS Floyd L. Steen, Lansdowne, Pa., assignor to General Electric Company, a corporation of New York Filed Mar. 20, 1968, Ser. No. 714,605 Int. Cl. H01f 40/00, 27/34, 27/28 US. Cl. 336174 5 Claims ABSTRACT OF THE DISCLOSURE This invention relates to a current transformer and, more particularly, to a current transformer having an accuracy substantially unaffected by stray [flux from high current buses adjacent the transformer.

The usual current transformer comprises a primary conductor carrying the current to be measured, a core surrounding the primary conductor, and a secondary winding wound about the core in which there is induced a secondary current representative of the measured primary current. If such a current transformer is applied in the vicinity of a bus carrying large currents through a path adjacent the core, the transformer accuracy may be seriously impaired by stray flux from the adjacent bus.

An object of my invention is to provide simple, inexpensive and effective means for maintaining the accuracy of the transformer substantially unimpaired by such stray flux from an adjacent bus.

In carrying out my invention in one form, I provide a core of nonmagnetic, electrical insulating material substantially surrounding the conductor carrying the current that is to be sensed. A secondary conductor is wound about the core in an even number only of successive layers. Assuming that the layers are consecutively numbered from the core outward beginning with the number 1, each odd numbered layer extends from one point on the core along substantially the entire core length to a second point on the core closely adjacent the first point. Each even numbered layer extends from the second point along substantially the entire core length back to substantially said first point. Terminals are respectively connected to the first and last of these layers at said first point. The turns in the adjacent layers are wound about the core in the same direction, and all the layers have a substantially equal number of turns distributed substantially uniformly along the length of the core.

For a better understanding of the invention reference may be had to the following description taken in conjunction with the accompanying drawing, wherein:

FIG. 1 is a cross sectional view of an electric power system including a current transformer embodying one form of the present invention.

FIG. 2 is a sectional view along the line 2-2 of FIG. .1.

United States Patent 0 FIG. 3 is a schematic view of a portion of my current transformer.

Referring now to FIG. 1, there are shown two closelyspaced bus bars 10 and 12 which are intended normally to carry high alternating currents, e.g., currents as high as several thousand amperes. Under fault conditions, the current through one or both of these bus bars can increase to values as high as 20 or 30 times this current.

For measuring the current through conductor 10 under any of these current conditions, I provide a current transformer 20. This current transformer comprises a toroidal core 22 suitably mounted in a position surrounding conductor 10 and a secondary winding 24 wound about the core. Conductor 10 can be considered as the primary conductor of the transformer. The core 22 is made of a material which is of constant permeability regardless of flux level or direction. Preferably, this material is a nonconductive, nonmagnetic material, e.g., a suitable resin. The terminals of the secondary winding, which are located at point A, are designated 25 and 26.

The secondary winding 24 is Wound about the core in a plurality of layers, each having a substantially equal number of turns distributed substantially uniformly about the core along the length of the core. The first layer is formed by winding the secondary conductor about the core beginning at a point A and extending over substantially the entire length of the core to a point B located closely adjacent point A. A second, or next succeeding, layer is formed by winding the secondary conductor around the outer periphery of the first layer, beginning at point B and extending back along substantially the entire length of the core to point A.

The direction of winding of the turns in the first layer is the same as the direction of winding of the turns in the second layer. In other words, if it is assumed that the secondary conductor in the first layer is wound counterclockwise about the periphery of the core as considered in the transverse cross-sectional view of FIG. 2, then the secondary winding in the second layer is also wound counterclockwise about the core periphery. The arrows in FIG. 2 depict the direction that the conductor is wound in each layer. If additional layers of secondary winding are applied, these are also wound in the same direction about the core periphery, as viewed in transverse cross section. Only an even number of layers are used, and the winding pattern is repeated for each succeeding pair of layers. That is, each odd numbered layer extends from A to B, and each even numbered layer extends from B back to A (assuming the layers are counted from the innermost layer outward).

In accordance with well recognized current transformer principles," primary current through the conductor 10 will induce a current in secondary winding 24 that is substantially proportional to the primary current. No inaccuracies Will arise from core saturation or eddy current effects since the core, as described hereinabove, is of a nonmagnetic, insulating material.

The main problem that I am concerned with is reducing the inaccuracies caused by stray flux from the adjacent high current conductor 12 traversing the secondary winding of the current transformer. It can be shown that the current that will be induced by such stray flux in a given layer of secondary winding is proportional to the area enclosed by the conductor of that layer. For example, in the diagrammatic representation of FIG. 3, which shows a single layer of the secondary winding, the current that will be induced by the stray flux in this layer is proportional to the shaded area, which is the area enclosed by the secondary winding in this layer. This proportional relationship exists irrespective of the angle or direction at which the stray flux enters the shaded area. The current induced by stray flux in the second layer of secondary winding of FIG. 1 will be in an opposite direction to that induced in the first layer, considered relative to the termi- 4D nals of the secondary. By making the area enclosed by the second layer substantially equal to that enclosed by the first layer, I can make the current induced in the second layer substantially equal to that induced in the first layer, thereby reducing to substantially zero the net current induced by stray flux in the two layers.

1 am able to make the area enclosed by the second layer substantially equal to that enclosed by the first layer because I use a substantially equal number of uniformlydistributed turns in the two layers. The only difference in the respective areas enclosed by the two layers is the very slight difference resulting from the larger eifective core cross-section that the second layer is wound about. In this respect, note (in FIG. 2) that the first layer is Wound about the bare core but the second layer is wound about the core plus the first layer. Since the first layer is typically only about five mils in thickness, it will be apparent that the resulting difference in enclosed areas for a typical core (several inches or more in diameter) is very small and not usually significant.

If third and fourth layers are used, these will likewise enclose areas of substantially the same size, differing only by the very slight amount resulting from the presence of the third layer beneath the fourth layer.

The nonmagnetic, insulating nature of the core also helps to maintain the accuracy of the current transformer despite large amounts of stray flux in the vicinity of the core. Such stray flux could produce local saturation, nonlinearity in permeability, and eddy current effects in a similarly-located iron core, but these effects are eliminated in my core due to its nonmagnetic, insulating nature In the schematic illustrations of the drawing, each layer of the secondary winding has relatively few turns. In one specific practical embodiment of the invention, each layer has over 400 turns on a core 3 inches in diameter. As a result, the conductor at each turn is disposed almost radially with respect to the center of the core, departing from a radius by less than one degree.

While I have shown and described a particular embodiment of my invention, it will be obvious to those skilled in the art that various changes and modifications may be made without departing from my invention in its broader aspects; and I, therefore, intend in the appended claims to cover all such changes and modifications as fall within the true spirit and scope of my invention.

What I claim as new and desire to secure by Letters Patent of the United States is:

1. A current transformer for sensing the current through one of a plurality of closely adjacent high current buses, comprising:

(a) a core of nonmagnetic electrical insulating material substantially surrounding said one bus,

(b) a secondary conductor wound about said core in an even number only of successive layers, the layers being consecutively numbered from the core outward, beginning with the number 1,

(c) each odd numbered layer extending from one point on said core along substantially the entire core length to a second point on the core closely adjacent said first point,

(d) each even numbered layer extending from said second point along substantially the entire core length back to substantially said first point,

(e) said secondary conductor comprising terminal portions respectively connected to the first and last of said layers at substantially said first point,

(f) the turns in any one layer being wound about the periphery of said core in the same direction as the turns of the next layer immediately thereadjacent, as considered in a transverse cross sectional plane through said core,

(g) all of said layers having a substantially equal number of turns distributed substantially uniformly along the length of said core.

2. The current transformer of claim 1 in which the total number of layers in said secondary winding is two.

3. The current transformer of claim 1 in which the total number of layers in said secondary winding is four.

4. The current transformer of claim 1 in which the area enclosed by the secondary conductor in said first layer is substantially equal to the area enclosed by the secondary conductor in said second layer.

5. The current transformer of claim 1 in which the area enclosed by the secondary conductor in any odd numbered layer is substantially equal to the area enclosed by the secondary conductor in any even numbered layer immediately adjacent said odd-numbered layer.

References Cited UNITED STATES PATENTS 2,241,127 5/1941 Harder 336-229 2,975,384 3/1961 Geiser 336229 XR 3,299,384 1/1967 Lee 336229 XR 3,353,132 11/1967 DEntremont 336229 XR FOREIGN PATENTS 419,329 2/ 1967 Switzerland.

LEWIS H. MYERS, Primary Examiner.

T. J. KOZMA, Assistant Examiner.

U.S. Cl. X.R. 

